The thermo-chemical conversion of biomass material by steam treatment has been described in U.S. Pat. No. 4,597,772, which is incorporated herein by reference, and the articles disclosed in the background portion of the '772 patent. The term "biomass material" is defined in the '772 patent and includes solid waste, paper, wood pulp, sawdust, wood chips, and the like.
Most biomass gasification processes are derived from coal technology, and utilize partial oxidation, along with pyrolysis, to convert biomass material into gaseous fuel. If air is used, the fuel gas is "low Btu," about 175 Btu/scf, because of the high nitrogen content in air acting as a diluent. If oxygen-blown, the fuel gas produced is "medium Btu," about 350 Btu/scf.
Synthetic natural gas (SNG) is essentially all methane, and has a heating value of about 1,000 Btu/scf. SNG cannot be made from low Btu gas because of the difficulty in removing nitrogen. SNG can be produced from medium Btu gas. Such gas, however, contains a relatively high amount of carbon monoxide, and a "water gas shift" is required in the process to bring the H.sub.2 /CO ratio to 3/1 for methanation. This factor, as well as the cost of oxygen, has discouraged consideration of oxygen-blown gasification in making SNG from biomass.
The feeding mechanism of the '772 patent is best shown in FIGS. 1, 6, 7, 8 and 9 of the '772 patent. The feeding system of the '772 patent serves to feed a fixed volume of biomass material, at atmospheric pressure, into the interior of the kiln, which operates at pressures on the order of 300 psi or greater. This is accomplished by supplying biomass material through an angled inlet pipe into a cylinder, within which a ram head reciprocates. A motor-driven valve is disposed below the input pipe, and a valve system serves to equalize the pressure or create a pressure differential on either side of the motor-driven valve. A problem with the feeding system of the '772 patent is that the motor-driven valve does not seal properly over time because of the significant pressure differential of 300 psi acting on the valve. Typically, some biomass material may get caught in the valve, thereby causing it to lose its sealing integrity.
Similarly, the discharge system of the '772 patent could be susceptible to leakage. The discharge system permits the removal of solid residue, which has not been converted to gas, at the output end of the kiln. Although it is anticipated that steam gasification of certain biomass material will be effective to completely convert the biomass material to gas, the gasification process for most biomass material will be about 98-99%. Thus, the small amount of solid residue must be removed periodically. According to the '772 patent as shown in FIGS. 1 and 2, a discharge conduit leads from the kiln, and two stop valves are disposed in the discharge conduit. The first stop valve is opened to permit the residue to move to the portion of the discharge conduit between the two stop valves. Subsequently, the first stop valve is closed and the second stop valve is opened so that the residue is discharged from the discharge conduit into a collection pan.
The vanes used to mix the biomass material are curved, as shown in FIG. 3 of the '772 patent. These curved vanes are difficult to construct, and tend to straighten out over time thereby causing jamming. Also, biomass material tends to lie in the concave portion of each vane, for perhaps a quarter of a turn, where the material undesirably may be heated too quickly.
Accordingly, it would be desirable to provide a feeding system for the gasifier of the '772 patent which is capable of delivering a fixed volume to the kiln without leakage. Furthermore, it would be desirable to provide a discharge system for the gasifier of the '772 patent which is capable of permitting the solid residue to be removed from the kiln without leakage. In addition, improvements relating to the rotor and the intake and exhaust manifolds of the gasifier of the '772 patent would be desirable.